This application claims the priority of German priority document 100 29 768.4, filed Jun. 16, 2000, the disclosure of which is expressly incorporated by reference herein.
The present invention is directed to a method for monitoring and maintaining a geostationary satellite in a correct orbital location.
Geostationary satellites must meet high precision demands with respect to their positioning. A typical value for the tolerance range in the longitude and latitude is xc2x10.05 degrees. Due in particular to the perturbational effect of external disturbances acting upon the satellite, orbital maneuvers must be carried out in order to maintain the satellite position in the defined tolerance range. A distinction must be made in this regard between maintaining a position predominantly in the north-south direction (as known, for example, in U.S. Pat. Nos. 5,020,746 and 5,826,831), and predominant east-west positioning (as known, for example, from T. J. Kelly et al., xe2x80x9cStationkeeping of Geostationary Satellites with Simultaneous Eccentricity and Longitude Controlxe2x80x9d, Journal of Guidance Control and Dynamics, Vol. 17,. No. 4, July-August 1994, and from E. M. Soop, xe2x80x9cHandbook of Geostationary Orbitsxe2x80x9d, Chapter 7.1-7.4, Space Technology Library, 1994). The significant disturbances for the position of the satellite are:
Radiation Pressure of the Sun
The radiation pressure of the sun influences the eccentricity (also known as vector eccentricity) of the satellite orbit which causes a correction requirement, particularly in the east-west direction. Particularly for satellites with large solar generators, such as communication satellites, solar pressure is the dominant disturbance for the east-west movement.
Higher Terms in the Gravitation Potential of the Earth
The tessieral terms of the earth""s gravitation potential cause an acceleration {umlaut over (xcex)} in the mean geographical longitude of the satellite. Such acceleration depends on the geographical longitude xcex of the satellite. At four points over the earth, it has a zero point (point of equilibrium) and, at other points, it reaches a maximal value |{umlaut over (xcex)}|max=2xc2x710xe2x88x923 degrees/day2 .
Gravitation Effects of Sun and Moon
These interference effects act predominantly in the north-south direction. For the east-west direction, they cause periodic interference effects in the mean longitude and in the vector eccentricity.
It is an object of the present invention, to provide an effective method for maintaining the position of geostationary satellites which requires only minimal control expenditures and therefore minimal fuel expenditures.
This and other objects and advantages are achieved by the method according to the invention, for maintaining the position of geostationary satellites and for compensating the influence of interferences on the position of the satellite, in which a determination is made of the range of the satellite orbit (relative to a space-fixed reference system) the perturbational effect of external interferences on the satellite position reaches a maximum. Actuators are activated in this range of maximal influence to maintain the satellite position. Therefore, according to the invention, it is not simply determined reactively whether the present position of the satellite exceeds a certain tolerance range so that subsequent control can be implemented to correct its position; rather, the orbit range is actively determined in which the maximal influence takes place, and this influence is counteracted already at the point and time of its occurrence.
That is, according to the invention, correction maneuvers are carried out not only when the satellite position exceeds a certain tolerance range. Instead interfering influences are corrected in the ranges on the satellite orbit which are optimal for the existing actuator configuration, even if no departure from the tolerance range has occurred. The method thus permits a reduction of the tolerance range while the fuel requirement remains the same and the tolerance range is kept constant.
The method according to the invention is particularly useful for the compensation of interfering influences in the east-west direction. For this purpose, actuators are used which act predominantly tangentially with respect to the satellite orbit.
The method according to the invention can also be used, however, for north-south position controls as well as for east-west position controls.
Although any one or more of the above-mentioned interferences can be considered to be disturbances for present purposes, it is sufficient to focus consideration on the dominant disturbance for the respectively existing application case. Thus, for example, for corrections predominantly in the east-west direction, the radiation pressure of the sun can be considered the dominant disturbance, but it can also become relevant to other cases.
If the radiation pressure of the sun is considered a disturbance, the actuators preferably are activated in a range of the satellite orbit whose right ascension (xcex1m) encloses an angle of between 45xc2x0 and 135xc2x0, particularly 90xc2x0, with the right ascension of the sun (xcex1s) in a geocentrically equatorial reference system. These are the ranges of the satellite orbit in which the interfering influence of the solar pressure becomes maximal. Thus, specifically in these ranges of the satellite orbit, it is advantageous to activate the actuators as a countercontrol in order to largely counteract a positional deviation of the satellite.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.